1. Field of the Invention
This invention generally relates to microelectromechanical systems (MEMS) displays and, more particularly, to a system and method for full color range interferometric modulation.
2. Description of the Related Art
Recently, MEMS reflective displays have been developed using interferometric light modulation three subpixel (red, green, and blue (RGB)) devices. Advantageously, these displays do not require backlighting. Other colors are generated by mixing of these three primary colors. Moreover, grayscale images can be generated using spatial or temporal addressing of the three subpixels. However, since each pixel is divided into three subpixels, the total reflectance for a primary color can be no more than 33%. It would be much more desirable if a single pixel could generate all colors with 100% reflectivity.
FIG. 1 is a MEMS light valve switch expressed as a spring-damping-capacitor model (prior art). Using parallel-plate MEMS technology, a color can be tuned to a desired reflection by applying a voltage between the reflective movable plate and a transparent fixed-position actuation electrode by varying the air gap distance. However for electrostatic force, a non-linear state known as “pull-in” effect can occur at approximately one-third the air gap distance, where the movable plate snaps down to the actuating electrode. This effect limits the tuning range to less than one-third the gap distance, which is the reason three subpixels are conventionally required to generate the three primary colors.
When a voltage is applied to each side of the parallel-plate capacitor, the movable plate is pulled toward the bottom plate by attraction of Coulomb force:
      F    elec    =            CV      2                      (                  g          -          d                )            2      
where C is the capacitance area, V is the applied voltage, g is the initial gap, and d is the displacement distance. At sufficiently small displacements, the deflection reaches an equilibrium position due to opposing Hooke's Law:Fmech=kd
However, when the displacement of the movable plate is larger than one-third the initial gap, i.e. d>g/3, the Hooke's force is not strong enough to balance the Coulomb force attraction. Therefore at this point, known as the pull-in voltage, the movable plate eventually snaps down to the non-equilibrium state.
The pull-in voltage is expressed as the following:
      V          pull      ⁢              -            ⁢      in        =                    8        27            ⁢                        kg          3                          ɛ          ⁢                                          ⁢          A                    
where ∈ is the electrical permittivity of the material, and A is the area of the parallel-plate capacitor.
It would be advantageous if a MEMS parallel plate reflective display device could be tuned over a wider range of colors.
It would be advantageous if the gap of a MEMS parallel plate display device could be continuously tuned over a large distance using a wide range of analog control voltages without reaching the pull-in voltage.